Rapid advancements are projected over the next decade to new materials for use in the interconnects: from aluminum to copper to reduce the resistance of the metal wires, and from SiO2 to dielectrics with lower dielectric constant k, commonly known as ‘low-k’ materials, for reduction of delay times on interconnect wires and minimization of crosstalk between wires. As stated in Semiconductor Industry Association, International Technology Roadmap for Semiconductors, Austin, Tex.: International SEMATECH, 2003, “The introduction of new low-k dielectrics . . . provide[s] significant process integration challenges”, and “ . . . will greatly challenge metrology for on-chip interconnect development and manufacture”. In addition, “design of interconnect structures requires measurement of the high frequency dielectric constant of low-k materials. High frequency testing of interconnect structures must characterize the effects of clock harmonics, skin effects, and crosstalk”. In general, such methodology should be non-destructive, non-contaminating, and provide real time/rapid data collection and analysis. (J. Iacoponi, Presented at the Characterization and Metrology for ULSI Technology conference, Austin, Tex., are performed on simplified structures or monitor wafers and are often destructive.
Currently, the two techniques most commonly used for dielectric constant measurements on blanket wafers are the Hg-probe and MIS-capacitor (D. K. Schroder, Semiconductor Material and Device Characterization, John Wiley & Sons, New York (1998)). Although widely employed, the Hg-probe contaminates the wafer, requires daily recalibration, is difficult to align, and cannot be used on production wafers. MIS-cap is considered more accurate than the Hg-probe, however it requires electrode patterning (e.g. destructive) and cannot be used on product wafers. The Corona discharge and Kelvin probe technique (D. K. Schroder, Meas. Sci. Technol. 12, R16 (2001)) which has traditionally been used to measure gate dielectrics is now being employed for low-k interconnect dielectrics, however, it is limited to blanket monitor wafers and there is some concern that the charging of the film surface may cause damage.
Near-field scanning microwave microscopy has proven to be a promising methodology for highly localized measurements.
One of the main goals of the near-field scanning microwave microscopy is to quantitatively measure a material's dielectric constant with high sensitivity of lateral and/or depth selectivity (i.e. to determine the material's property over a small volume while ignoring the contribution of the volume's surrounding environment). This is particularly important in measurements on complex structures, such as semiconductor devices or composite materials, where, for example, the permittivity of one line or layer must be determined without having knowledge of the properties of the neighboring lines or underlying layers.
In order to perform highly localized quantitative measurements of a material's complex permittivity at microwave frequencies by means of near-field microwave microscopy the near-field probe requires calibration. All calibration procedures currently in use for near-field microwave microscopy employ some information about the actual tip geometry which would include, for example, the tip curvature radius, etc., and further requires knowledge of the absolute tip-to-sample separation.
If there is no radiation from the tip of the probe, the response of the electrical near-field probe depends on the fringe impedance of the tip Zt=1/iωCt, where Ct is the static capacitance of the tip of the probe. This capacitance depends on the physical geometry of the tip, the tip-to-sample separation d, and the sample's dielectric constant k (assuming the sample is uniform in contour). Thus, in order to extract the sample's dielectric constant k from the impedance of the tip Zt, the tip geometry and absolute tip-to-sample separation must be known to a high degree of accuracy.
However, accurate determination of these parameters is difficult and often impractical, especially for small tips of less than or on the order of a few microns in size which are of great importance for near-field microwave microscopy. Further, analytical solutions to the problem of interaction between a near-field tip and a sample exist only for the most simple tip geometries, such as a sphere or a flush end of a coaxial line.
It is therefore highly desirable to perform quantitative measurement of a material's dielectric constant which does not require knowledge of either the actual tip geometry or the absolute tip-to-sample separation.
Accurate and precise novel measurement approach with the use of near-field microwave probe is needed to obtain a material's dielectric constant.